Known in the past has been a choke transformer for a power circuit which is comprised of a plate-shaped magnetic core (hereinafter referred to simply as a “core”), a coil which is wound around the core, and two terminals which are formed at the two end parts of the surface of the core and which are electrically joined to the two ends of the coil (for example, see Japanese Laid-Open Patent Publication No. 11-243021). In this choke coil, the lead wires are connected to the terminals by high temperature solder. The terminals are bonded to the surface of the core by a conductive binder so that the lead wires are interposed between the terminals and the core.
In recent years, the spread of the Internet, digital TV, etc. has led to the use of pulse transformers as transformer modules for efficiently transmitting pulse signals which are handled by the digital circuits. In particular, LAN interface devices which are mounted in PCs and other information devices and in AV equipment combining video and audio use pulse transformers for the purpose of insulation and noise elimination. A pulse transformer is configured the same as a power transformer which is designed for voltage conversion and comprises a core around which a primary side and secondary side windings are wound. These are insulated and not electrically joined. In a pulse transformer, a signal is transmitted by magnetic coupling. Voltage is induced proportional to the number of windings in the same way as a power transformer.
As illustrated in FIGS. 1A and 1B, a transformer module 2 has transformers 10 which are mounted in a case 20 of a type with external terminals 30 which are formed in gull wing shapes (L-shapes) taken out from the two side surfaces. The case 20 in which the transformers 10 are mounted has a size of a vertical 10 mm, horizontal 18 mm, and height 2 mm or so. The transformers 10 which are mounted inside it have cores 12 of sizes of diameters of 2 to 4 mm or so.
When connecting the windings 14 which are wound around the cores 12 of the transformers 10 which are carried in the case 20 to the external terminals 30, as illustrated in FIG. 1B, the end parts (hereinafter called “leads”) 60 of the windings 14 are generally wound around the winding parts 32 of the external terminals 30 and then joined there by solder etc. Note that, FIG. 1B omits the illustration of the solder for joining the leads 60 of the windings 14 which are wound around the winding parts 32 of the external terminals 30 to the winding parts 32. When winding the leads 60 of the windings 14 around the winding parts 32 of the external terminals 30, the leads 60 of the windings 14 are wound around the winding parts 32 in a tensed state, so there is no longer any slack in the leads 60 of the windings 14 and tensile force is generated.
In the transformer module 2 which is illustrated in FIG. 1B, if the transformer module 2 is mounted on a circuit board with the tensile force which is generated at the leads 60 of the windings 14 which are wound around the cores 12 maintained as it is, for example, in the reflow process of the solder, the leads 60 of the windings 14 will thermally expand and stretch due to the heat of the solder. In this regard, if the heat is cooled off after the transformer module 2 is mounted on the circuit board, the expanded leads 60 of the windings 14 will thermally contract and shrink and may cause the leads 60 to break.